The function of the catalytic cracking unit is to convert heavy straight-run distillate fractions into lighter and more useful liquids and gases by high temperature pyrolysis in the presence of a catalyst. Typical flow schemes and operating conditions are given in Reference 1. These reactions ordinarily occur at pressures of 10 to 15 psig and at temperatures in the range of 1,000 degrees F. The reactor effluent stream contains a full range of products from hydrogen through coke. At the entrance to the fractiona-tor, it is a superheated vapor 2nd must be cooled and simultaneously fractionated in carefully controlled stages to yield products having the desired properties.

The problems and calculational techniques involved in catalytic fractionator design are quite similar to those encountered in crude distillation, and, thus, the procedures given are somewhat less detailed to avoid useless repetition. The principal areas of difference between the two systems are as follows.

1. The feed to the tower is a superheated vapor, not a vapor-liquid mixture.

2. The heaviest materials must be condensed out and yielded as a bottoms liquid to prevent contamination of the first sidedraw liquid.

3. The sidedraw products usually have much wider boiling ranges than do the corresponding products from a crude unit. The properties of these streams are discussed in a later portion of this chapter.

The principal areas of similarity in the two systems are as follows.

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